1. Field of the Invention
The present invention relates to an electron microscope, especially of a type capable of functioning also as a scanning type electron microscope.
2. Description of Prior Arts
Recently, various attempts have been made to provide a normal electron microscope with a function of scanning type electron microscope.
In one example of such attempts, a specimen is disposed within a magnetic field of an objective lens capable of producing an intense magnetic field.
An electron beam is focussed on the specimen by the magnetic field distributed over an area at the incident side of the specimen, which magnetic field is produced by the objective lens and referred to as "fore magnetic field". At the incident side of the fore magnetic field, i.e. at one side of that field closer to an electron gun, there is provided a deflector for deflecting the focussed electron beam so that the specimen is scanned with the focussed electron beam two-dimensionally. Although this type of microscope has a high resolution, due to the short focal length of the lens formed by the fore magnetic field, the magnetic field is inconveniently disturbed resulting in an astigmatism, when the specimen is ferromagnetic, to deteriorate the resolution unfavourably.
In addition, the depth of focus is small, due to an extremely large convergent angle of the incident electron beam, so that a field of three dimensions as can be obtained in the usual scanning microscope's secondary electron image is not obtainable.
Another disadvantage is that the size of the specimen is inevitably limited by the size of the magnetic pole of the objective lens.
In another attempt for imparting the scanning function to the electron microscope, the specimen is positioned not in the magnetic field of the objective lens but at the emitting side of the objective lens, as is the case of usual electron microscope specifically designed for scanning.
In this case, the electron beam is focussed on the specimen by a relatively less intense excitation of the objective lens, while the specimen is scanned with the focussed electron beam which is deflected by a deflector disposed at incident side of the specimen.
Although this type of microscope can overcome above described shortcomings inherent in the first mentioned example, the resolution of the scanning electron microscope is largely decreased, due to an increased coefficient of spherical abberation. This is due to the fact that the distance between the objective lens and the specimen is large, partly because the upper and the lower magnetic poles for forming objective magnetic field are made to project closer to the axis of the beam than other portion of the objective lens, and partly because the projecting portions are not located at the emitting end of the objective lens.
Under these circumstances, the present inventors have noted that the distance between the objective lens and the specimen can be decreased by positioning the magnetic field of the objective lens at the emitting end of the objective lens, through forming the lower magnetic pole of the objective lens in a flat plate-like shape.
However, in usual electron microscope, in order to preserve its performance, the objective lens must have the value of IN/.sqroot.E (IN denotes ampere turn, and E means acceleration voltage) as large as 15 to 16.
Thus, the study by the present inventors has been focussed on whether the above value is met or satisfied, when the objective magnetic field is positioned at the emitting end of the objective lens by making the lower pole in the flat plate-like form.
As a result, it has been concluded that the IN/.sqroot.E value as large as 15 to 16 is not obtainable, due to a magnetic saturation of the objective lens.